The invention relates to a heat dissipation device, and in particular to a heat dissipation device reducing a reset current of fan motor when the fan motor is locked.
Information is rapidly exchanged by electronic devices. A notebook is given as an example. When a notebook transmits a large amount of data, a central processing unit (CPU) therein generates excessive heat due to the data transmission. Thus, a notebook requires an effective heat dissipation device with low power consumption to dissipate heat.
FIG. 1A is a block diagram of one conventional heat dissipation device for a CPU. FIG. 1B shows the relationship between a duty cycle of a pulse width modulation (PWM) signal and a rotation rate of a fan motor in FIG. 1A. Referring FIG. 1A, a heat dissipation device 10 comprises a fan motor 12, a drive circuit 14 coupled to the fan motor 12, and a digital/analog (D/A) converter 16 coupled to the drive circuit 14. When the heat dissipation device 10 performs a heat dissipation process for a CPU 18, a PWM signal is first input to the heat dissipation device 10. After receiving the PWM signal, the D/A converter 16 converts it from a digital voltage signal to an analog voltage signal. The drive circuit 14 then outputs a rotation signal according to the analog voltage signal for controlling a rotation rate of the fan motor 12. After receiving the rotation signal, the fan motor 12 performs the heat dissipation process for the CPU 18. Referring to FIG. 5, a first reference voltage Vref1 is larger than a second reference voltage Vref2. When the analog voltage signal from the D/A converter 16 to the drive circuit 14 is lower than the second reference voltage Vref2, the fan motor 12 is in a full rotation state (at high rotation rate). When the analog voltage signal is between the first reference voltage Vref1 and the second reference voltage Vref2, the fan motor 12 has a variable rotation rate. When the analog voltage signal is higher than the first reference voltage Vref1, the fan motor 31 is in a half rotation state (at low rotation rate). As shown in FIG. 1B, when the amount of heat generated by the CPU 18 is increased, the duty cycle of the PWM signal input to the heat dissipation device 10 is increased. Thus, the voltage value of the analog voltage signal from the D/A converter 16 is decreased, and the fan motor 12 is driven by a increased current value, so that the rotation rate of the fan motor 12 is increased.
In general, when the fan motor is locked at a certain rotation rate, the greater the duty cycle of the PWM signal is, the larger a reset current required by the fan motor 12 is. For example, when the fan motor 12 is at a low rotation rate 1500 rpm, the duty cycle of the PWM is 0%, and the reset current required by the fan motor 12 in the locked state is 0.2 A. When the rotation rate of the fan motor 12 is 2500 rpm, the duty cycle of the PWM is 50%, and the reset current required by the fan motor 12 in the locked state is increased to 0.3 A. When the rotation rate of the fan motor 12 is increased to 3500 rpm, the duty cycle of the PWM is 100%, and the reset current required by the fan motor 12 in the locked state is increased to 0.5 A.
Under the three conditions described, although the CPU 18 can have a great capacity for dissipating heat, the reset current required by the drive circuit 14 and the fan motor 12 is increased, resulting in the increased amount of waste heat generated by the drive circuit 14. The temperature of the coils of the fan motor 12 is further raised. This seriously decreases the life of the heat dissipation device 10 and may even damage the heat dissipation device 10.
FIG. 2A is a block diagram of another conventional heat dissipation device for a CPU. FIG. 2B shows the relationship between a temperature detected by a temperature-controlled circuit and a rotation rate of a fan motor in FIG. 2A. Referring FIG. 2A, a heat dissipation device 20 comprises a fan motor 22, a drive circuit 24 coupled to the fan motor 22, and a temperature-controlled circuit 26 coupled to the drive circuit 24 for detecting the ambient temperature of the CPU 18. When the heat dissipation device 20 performs a heat dissipation process for the CPU 18, the temperature-controlled circuit 26 first detects the ambient temperature of the CPU 18 and outputs a voltage signal to the drive circuit 24 according to the ambient temperature. The drive circuit 24 outputs a rotation signal to control the rotation rate of the fan motor according to the voltage signal. As shown in FIG. 2B, when the heat generated by the CPU 18 is increased, the ambient temperature detected by the temperature-controlled circuit 26 is raised. The voltage value of the rotation signal from the drive circuit 24 is decreased, and the current value thereof is increased incrementally, so that the rotation rate of the fan motor 22 is continuously increased to a limit rate.
In general, the rotation rate of the fan motor 22 stays at the lowest rotation rate (half rate) when the ambient temperature detected by the temperature-controlled circuit 26 is lower than a first predetermined value, and a reset current required by the fan motor 22 is the lowest. The rotation rate of the fan motor 22 stays at the greatest rotation rate (full rate) when the ambient temperature detected by the temperature-controlled circuit 26 is higher than a second predetermined value, and the reset current required by the fan motor 22 is the greatest one.
For example, when the CPU 18 processes less data, the heat generated by it is lower, and the ambient temperature detected by the temperature-controlled circuit 26 is relatively lower. If the fan motor 22 is to be locked in this condition, the value of the voltage signal from the temperature-controlled circuit 26 to the drive circuit 24 is the largest (assumed as 5V), and a current value from the drive circuit 24 to the fan motor 22, which is the reset current required by the fan motor 22, is the smallest (assumed as 0.2 A). At this time, the rotation rate of the fan motor 22 is 1500 rpm. When the amount of data processed by the CPU 18 is increased, the ambient temperature detected by the temperature-controlled circuit 26 is raised. Under 20° C., the value of the voltage signal from the temperature-controlled circuit 26 to the drive circuit 24 is decreased with the raised ambient temperature, the rotation rate is still 1500 rpm, and the reset current of the fan motor 22 is still 0.2 A. When the ambient temperature detected by the temperature-controlled circuit 26 is in the range between 20° C. and 40° C. and the fan motor 22 is to be locked in this condition, the value of the voltage signal output from the temperature-controlled circuit 26 to the drive circuit 24 varies between 3V and 1V. At this time, the reset current of the fan motor 22 in the locked state is changed between 0.2 A and 0.5 A, so that the rotation rate of the fan motor 22 is in the rage between 1500 rpm and 3500 rpm. When the amount of data processed by the CPU 18 is much greater, the ambient temperature detected by the temperature-controlled circuit 26 is higher than 40° C. If the fan motor 22 is to be locked in this condition, the value of the voltage signal output from the temperature-controlled circuit 26 to the drive circuit 24 is lowered below 1V. At this time, the reset current of the fan motor 22 in the locked state remains at 0.5 A, that is, the rotation rate of the fan motor 22 remains at 3500 rpm of full rate.
Under the above conditions, although the heat dissipation device 20 can provide the CPU 18 with a great capacity for dissipating heat, the reset current output from the drive circuit 24 to the fan motor is increased with the raised ambient temperature detected by the temperature-controlled circuit 26, resulting in the increased amount of waste heat generated by the drive circuit 24. Thus, the temperature of the coils of the fan motor 12 is raised, which seriously reduces the life of the heat dissipation device 10.